1. The Field of the Invention
The present invention relates to the field of high-speed data communication. In particular, the present invention relates to a circuit and method for filtering a laser signal.
2. Description of the Related Art
In high-speed communication systems, laser signals may be used to transmit information. For example, a laser signal (transmitted by a remote device, for example) may be detected by a photo detector, amplified by a trans-impedance amplifier circuit, and then filtered by a filter circuit. After the laser signal is filtered, it is then amplified by an amplification circuit and converted to a digital format for further processing and storage. Traditionally, a set of low pass filters are used to filter out noise from the laser signal and to filter out higher order harmonics created by the photo detector and the trans-impedance amplifier prior to the amplification stage. These low pass filters are typically implemented externally to an amplification integrated circuit (IC) by using inductors, capacitors, and resistors.
However, there are a few problems associated with the external low pass filters. The external components of the filter circuits occupy a significant amount of space on a circuit board, and they add significant costs to the system. Another problem with the external filter circuits is that once the circuit components are placed on a circuit board, the characteristics of the low pass filters may not be easily adjusted, because the sizes of the inductors, capacitors and resistors used to implement the low pass filters are fixed. However, in real world applications, a laser signal filter circuit needs to support of a wide spectrum of signal bandwidths, which requires the filter circuit to operate at different frequencies. For example, an external filter circuit may work well in a normal transmission rate. But the filter quality degrades when the frequency of the laser signal is increased to twice or four times the normal transmission rate. Yet another problem with the prior art filter circuits is that the capacitors used to implement the filter circuits may not be easily adjusted to compensate for deviations of circuit devices caused by semiconductor manufacturing process variations.
Therefore, there is need for reducing the costs associated with the circuit components of the external filter circuits. There is a need for minimizing the board space occupied by such external circuit components. In addition, there is a need for a laser signal filter circuit that supports a wide spectrum of laser signal bandwidth applications without degrading the quality of the filtered signal. Furthermore, there is a need for adjusting the characteristics of individual filters by tuning the capacitors within the filtered circuit in order to compensate for deviations caused by variations in the manufacturing of integrated circuit devices containing the filter circuit.